The present invention is directed generally to the recovery of precious metals from precious metal-containing material and specifically to the recovery of precious metals from precious metal-containing material using thiosulfate lixiviants.
A traditional technique for recovering precious metal(s) from precious metal-containing ore is by leaching the material with a cyanide lixiviant. As used herein, a xe2x80x9cprecious metalxe2x80x9d refers to gold, silver, and the platinum group metals (e.g., platinum, palladium, ruthenium, rhodium, osmium, and iridium). Many countries are placing severe limitations on the use of cyanide due to the deleterious effects of cyanide on the environment. Incidents of fish and other wildlife having been killed by the leakage of cyanide into waterways have been reported. The limitations being placed on cyanide use have increased substantially the cost of extracting precious metal(s) from ore, thereby decreasing precious metal reserves in many countries. Cyanide is also unable to recover precious metals such as gold from refractory ores without a pretreatment step. xe2x80x9cRefractory oresxe2x80x9d refer to those ores that do not respond well to conventional cyanide leaching. Examples of refractory ores include sulfidic ores (where at least some of the precious metals are locked up in the sulfide matrix), carbonaceous ores (where the precious metal complex dissolved in the lixiviant adsorbs onto carbonaceous matter in the ores), and sulfidic and carbonaceous ores.
Thiosulfate has been actively considered as a replacement for cyanide. Thiosulfate is relatively inexpensive and is far less harmful to the environment than cyanide. Thiosulfate has also been shown to be effective in recovering precious metals from pretreated refractory preg-robbing carbonaceous ores and sulfidic ores. As used herein, xe2x80x9cpreg-robbingxe2x80x9d is any material that interacts with (e.g., adsorbs or binds) precious metals after dissolution by a lixiviant, thereby interfering with precious metal extraction, and xe2x80x9ccarbonaceous materialxe2x80x9d is any material that includes one or more carbon-containing compounds, such as humic acid, graphite, bitumins and asphaltic compounds.
Where gold is the precious metal, thiosulfate leaching techniques have typically relied on the use of copper ions to catalyze and accelerate the oxidation of gold, ammonia to facilitate the formation and stabilization of cupric ammine ions and/or a pH at pH 9 or above to maintain a region of stability where both the cupric ammine and gold thiosulfate complexes are stable.
It is well known in the art that the catalytic effect of copper and ammonia in conventional thiosulfate leaching of gold is described by the following sequence of reactions. Formation of the cupric ammine complex:
Cu2++4NH3xe2x86x92Cu(NH3)42+xe2x80x83xe2x80x83(1)
Oxidation of gold by cupric ammine, gold complexation as the gold-thiosulfate anion, and reduction of the cupric ammine to cuprous thiosulfate:
Au+Cu(NH3)42++5S2O32xe2x88x92xe2x86x92Au(S2O3)23xe2x88x92+Cu(S2O35xe2x88x92+4NH3xe2x80x83xe2x80x83(2)
Oxidation of the cuprous thiosulfate back to cupric ammine with oxygen:
Cu(S2O3)35xe2x88x92+4NH3+xc2xcO2+xc2xdH2Oxe2x86x92Cu(NH3)42++3S2O32xe2x88x92+OHxe2x88x92xe2x80x83xe2x80x83(3)
Summing equations (2) and (3) yields the overall thiosulfate leach reaction for gold:
Au+2S2O32xe2x88x92+xc2xcO2+H2Oxe2x86x92Au(S2O3)23xe2x88x92+OHxe2x88x92xe2x80x83xe2x80x83(4)
It can be seen from the above equations that copper and ammonia act as catalysts in that they are neither produced nor consumed in the overall leach reaction.
Cupper and ammonia can be a source of problems. Added copper tends to precipitate as cupric sulfide, which is speculated to form a passive layer on gold, thereby inhibiting gold leaching as well as increasing copper and thiosulfate consumption:
Cu2+S2O32xe2x88x92+2OHxe2x88x92xe2x86x92CuS+SO42xe2x88x92+H2Oxe2x80x83xe2x80x83(5)
Rapid oxidation of thiosulfate by cupric ammine also occurs, leading to excessive degradation and loss of thiosulfate:
2Cu(NH3)42++8S2O32xe2x88x92xe2x86x922Cu(S2O3)35xe2x88x92+S4O62xe2x88x92+8NH3xe2x80x83xe2x80x83(6)
Loss of ammonia by volatilization occurs readily, particularly in unsealed gas-sparged reactors operating at pH greater than 9.2, leading to excessive ammonia consumption:
xe2x80x83NH4++OHxe2x88x92xe2x86x92NH3(aq)+H2Oxe2x86x92NH3(g)+H2Oxe2x80x83xe2x80x83(7)
Like cyanide, copper and ammonia are highly toxic to many aquatic lifeforms and are environmentally controlled substances.
Other problems encountered with thiosulfate leaching include difficulty in recovering gold out of solution as a result of the formation of polythionates, such as tetrathionate and trithionate, which adsorb competitively with gold onto adsorbents, such as resins. The formation of polythionates further increases thiosulfate consumption per unit mass of processed ore.
These and other needs have been addressed by the methodologies and systems of the present invention. The methodologies can recover precious metals from a variety of materials, including refractory carbonaceous or sulfidic ores, double refractory ores (e.g., ores containing both sulfide-locked gold and carbonaceous preg-robbing matter), oxide ores, nonrefractory sulfidic ores, and ores also containing copper minerals and other materials derived from such ores (e.g., concentrates, tailings, etc.).
In one embodiment, a thiosulfate leaching process is provided that includes one or more of the following operating parameters:
(a) an oxygen partial pressure that is preferably superatmospheric and more preferably ranges from about 4 to about 500 psia;
(b) a leach slurry pH that is preferably less than pH 9;
(c) a leach slurry that is preferably at least substantially free of (added) ammonia and more preferably contains less than 0.05M (added) ammonia such that the leach slurry has a maximum total concentration of ammonia of preferably less than 0.05M and more preferably no more than about 0.025M;
(d) a leach slurry that is preferably at least substantially free of (added) copper ion and more preferably contains no more than about 15 ppm (added) copper ions;
(e) an (added) sulfite concentration that is preferably no more than about 0.01 M such that the slurry has a maximum total concentration of sulfite of preferably no more than about 0.02M and more preferably no more than about 0.01M; and/or
(f) a leach slurry temperature preferably ranging from about 20 to about 100xc2x0 C. and more preferably from about 20 to about 80xc2x0 C.
The foregoing parameters can yield a high level of precious metal extraction from the precious metal-containing material, which can be at least about 70% and sometimes at least about 80%.
The thiosulfate lixiviant can be derived from any suitable form(s) of thiosulfate, such as sodium thiosulfate, calcium thiosulfate, potassium thiosulfate and/or ammonium thiosulfate. Sodium and/or calcium thiosulfate are preferred.
The leaching process can be conducted by any suitable technique. For example, the leaching can be conducted in situ, in a heap or in an open or sealed vessel. It is particularly preferred that the leaching be conducted in an agitated, multi-compartment reactor such as an autoclave.
The precious metal can be recovered from the pregnant leach solution by any suitable technique. By way of example, the precious metal can be recovered by resin adsorbtion methods such as resin-in-pulp, resin-in-solution, and resin-in-leach or by solvent extraction, cementation, electrolysis, precipitation, and/or combinations of two or more of these techniques.
Reducing or eliminating the need to have copper ions and/or ammonia present in the leach as practiced in the present invention can provide significant multiple benefits. First, the cost of having to add copper and ammonia reagents to the process can be reduced significantly or eliminated. Second, environmental concerns relating to the presence of potentially harmful amounts of copper and ammonia in the tailings or other waste streams generated by the process can be mitigated. Third, the near-absence or complete absence of copper and ammonia in the leach can provide for a much more reliable and robust leaching process, yielding more stable leachates, able to operate over a wider pH and oxidation-reduction potential (ORP) range than is possible with conventional thiosulfate leaching. The latter process must operate in the relatively narrow window of pH and ORP where both the cupric ammine complex and the gold thiosulfate complex co-exist. With the process of the present invention, the pH of the thiosulfate lixiviant solution in the leaching step can be less than pH 9 and the ORP less than 200 mV (referenced to the standard hydrogen electrode). Fourth, minimizing the amount of copper in the system can lead to increased loading of gold onto resins due to reduced competitive adsorption of copper ions. Resin elutions are also simplified as little, if any copper, is on the resin. Finally, the near-absence or complete absence of copper and ammonia in the leach can reduce or eliminate entirely a host of deleterious side reactions that consume thiosulfate and are otherwise difficult or impossible to prevent.
The elimination or near elimination of sulfite from the thiosulfate leach also can have advantages. Sulfite can depress the rate of dissolution of precious metal from the precious metal-containing material by reducing significantly the oxidation reduction potential (ORP) of the leach solution or lixiviant. As will be appreciated, the rate of oxidation of the gold (and therefore the rate of dissolution of the gold) is directly dependent on the ORP.
In another embodiment, an extraction agent is preferably contacted with a pregnant (precious metal-containing) thiosulfate leach solution at a temperature of less than about 70xc2x0 C. and more preferably less than about 60xc2x0 C. in the substantial absence of dissolved molecular oxygen to isolate the precious metal and convert polythionates in the pregnant leach solution into thiosulfate. In one configuration, the extraction agent is an adsorbent, such as a resin, which loads the precious metal onto the adsorbent. As used herein, an xe2x80x9cadsorbentxe2x80x9d is a substance which has the ability to hold molecules or atoms of other substances on its surface. Examples of suitable resin adsorbents include weak and strong base resins such as xe2x80x9cDOWEX 21Kxe2x80x9d, manufactured by Dow Chemical. In another configuration, the extraction agent is a solvent extraction reagent that extracts the precious metals into an organic phase, from which the precious metals can be later recovered. As will be appreciated, the detrimental polythionates decompose into thiosulfate in the substantial absence of dissolved molecular oxygen.
In yet another embodiment, the pregnant leach solution from a thiosulfate leaching step is contacted, after the leaching step, with a reagent to convert at least about 50% and typically at least most of polythionates (particularly trithionate and tetrathionate) into thiosulfate. The reagent or reductant can be any suitable reactant to convert polythionates into thiosulfate, with any sulfide, and/or polysulfide (i.e., a compound containing one or a mixture of polymeric ion(s) Sx2xe2x88x92, where x=2-6, such as disulfide, trisulfide, tetrasulfide, pentasulfide and hexasulfide) being particularly preferred. A sulfite reagent can also be used but is generally effective only in converting polythionates of the form SxO62xe2x88x92, where x=4 to 6, to thiosulfate. The sulfite, sulfide, and/or polysulfide can be compounded with any cation, with Groups IA and IIA elements of the Periodic Table, ammonium, and hydrogen being preferred.
In yet another embodiment, a precious metal solubilized in a solution, such as a pregnant leach solution or eluate, is electrowon in the presence of sulfite. In the presence of sulfite, the precious metal is reduced to the elemental state at the cathode while the sulfite is oxidized to sulfate at the anode. Sulfite is also believed to improve the precious metal loading capacity of the resin by converting loaded tetrathionate to trithionate and thiosulfate.
In yet another embodiment, the formation of polythionates is controlled by maintaining a (pregnant or barren) thiosulfate leach solution in a nonoxidizing (or at least substantially nonoxidizing) atmosphere and/or sparging a nonoxidizing (or at least substantially nonoxidizing) gas through the leach solution. As will be appreciated, the atmosphere or gas may contain one or more reductants, such as hydrogen sulfide and/or sulfur dioxide. The molecular oxygen concentration in the atmosphere and/or sparge gas is preferably insufficient to cause a dissolved molecular oxygen concentration in the leach solution of more than about 1 ppm and preferably of more than about 0.2 ppm. Preferably, the inert atmosphere (or sparge gas) is at least substantially free of molecular oxygen and includes at least about 85 vol. % of any inert gas such as molecular nitrogen and/or argon. By controlling the amount of oxidant(s) (other than thiosulfate and polythionates) in the atmosphere and/or (pregnant or barren) leach solution the rate or degree of oxidation of thiosulfates to form polythionates can be controlled.